Preparation of retinoic acid esters of phorbol derivatives

The synthesis of 12-O-retinoylphorbol-13-acetate (RPA), an incomplete tumor promoter (second stage promoter) is described. The preparation starts with phorbol-13-acetate-20-tritylether which is acylated by a carbodiimide method to yield its 12-retinoate. The latter is detritylated by acidic methanol to give RPA. Following an analogous procedure, the 4-methyl-ether of RPA is prepared from 4-O-methylphorbol-13-acetate-20-tritylether.